Most of the bridge failings caused by a shock such as an earthquake are due to the breakage or separation of members by the shock of collisions at the points of connection between the adjacent horizontal members or between the horizontal member and the vertical member in the bridge. This fact was confirmed in the Great Hanshin-Awaji Earthquake of 1995.
For the prevention of bridge failings, various methods have hitherto been adopted, including the formation of a slippage-preventive protrusion (i.e., bracket) or a bridge falling-preventive wall (i.e., safety wall) on the top of a vertical member or on the bottom of a horizontal member; the connection between the horizontal member and the vertical member by PC steel parts or anchor bars; and connection between the adjacent horizontal members by PC steel parts.
In the breakage or falling of bridges as previously investigated in the earthquake disasters of the past, there have been often found damage caused by vertical displacement to the bridge axis and damage probably caused by shock vibration. For this reason, most of the bridge falling-preventive construction now in practical use involves both connecting construction that can follow the vertical movement to the bridge axis and shock-absorbing construction with shock absorbers for absorbing or attenuating the shock vibration.
The shock absorber which has been used in the bridge of such shock-absorbing construction may include molded rubber parts characterized by good restitution. In the case where shock absorbers are disposed at very limited sites such as points of connection between the adjacent horizontal members or between the horizontal member and the vertical member, the use of molded rubber parts gives a limitation on the size of shock absorbers, leading to a deterioration in the shock-absorbing performance, which makes it difficult to obtain satisfactory effects on the prevention of breakage or falling of bridges against strong and shock vibration. The shock absorption may also be increased by the use of molded rubber parts made thicker or by the combined use of more than one molded rubber part, in which either case, however, the shock absorbers become large-sized, so that they are difficult to dispose at very limited sites, in addition to a steep rise in material costs and an increase in weight.
Some shock absorbers other than molded rubber parts have also been known, for example, metal springs, shock-attenuating friction members, and shock-attenuating hydraulic members. Metal springs, although they have excellent shock-absorbing performance, have an inevitable problem of rust formation; therefore, elaborate maintenance is needed after construction and, from a viewpoint of resistance to rust and weather, they are not suitable for use in the bridges to be constructed at locations exposed to salt water, such as coastal bridges and marine connecting bridges. In general, friction or hydraulic shock-attenuating members are structurally complicated and both much expensive and heavy, and they cannot keep their original performance without undergoing proper maintenance.
As the shock absorbers using molded resin parts, Japanese Patent Publication No. 61-12779/1986 discloses a technique for the improvement of shock-absorbing performance where hollow molded parts of a thermoplastic resin elastomer are provided with permanent strain by pre-compression in the axial direction. However, such molded resin parts, although they have improved ability to function as an elastic body, have poor performance of absorbing the energy of compression, so that they cannot be expected to have satisfactory shock-absorbing performance for use in the prevention of bridge falling caused by earthquakes or other factors.
The present inventors have developed a shock absorber formed from an elastic resin, comprising more than one arch-, dome-, or honeycomb-shaped member capable of causing deformation by compression, which are disposed on a perforated or non-perforated flat plate of an elastic resin, and thereby having cushioning properties; and they have proceeded with various studies to put such a shock absorber to practical use. This type of shock absorbers is suitable for some applications in which they are widely spread over the side wall of a road or the floor of a building to exhibit uniform cushioning performance over a wide area; however, they are difficult to adopt some applications in which they have to be disposed at limited sites such as points of connection between the adjacent horizontal members or between the horizontal member and the vertical member, and they cannot exhibit satisfactory shock-absorbing performance.
The shock absorbers in the bridge construction are often disposed in the vicinity of horizontal member-bearing portions on the vertical members; therefore, they should not become an obstacle to the maintenance works of the bearing portions, such as inspection, conservation, and repair. Therefore, they are required to be small-sized and lightweight, and have excellent shock-absorbing performance, i.e., higher absorption of energy of compression rather than reaction; however, the conventional shock absorbers as described above cannot meet these requirements.